Employers' contributions into company pension schemes on behalf of top-rate taxpayers will be taxed as a benefit in kind, the Treasury confirmed today.

Pension experts have warned the move could be a further nail in the coffin for final salary schemes, with high-earning company directors lacking the incentive to keep expensive schemes running. And it could put some high earners off saving for their retirement at all.

Chancellor Alistair Darling announced yesterday that people earning £150,000 or more would have to pay a new marginal rate of income tax – 50% – from next April, while the tax relief they will get on their own pension contributions will be cut on a tiered basis to a minimum of 20% for those earning £180,000 or more.

But scrutiny of the budget small print has revealed these taxpayers will also have to pay income tax on any benefit paid into the scheme on their behalf by employers.

The Treasury confirmed today that from April next year employers' contributions on behalf of top-rate taxpayers would be taxed. A spokesman said: "It's a benefit in kind. It's deferred salary and will be taxed at a flat rate of 20%."

However, the new rule will not extend to members of company pension schemes who earn less than £150,000.

This means a top-rate taxpayer would have to pay £2,000 in tax on a £10,000 contribution to a money purchase scheme. But the situation is even more complicated and expensive for final salary scheme contributions, where benefits are added on in the form of years. The tax bill could be much higher.

Tom McPhail of independent financial advisers Hargreaves Lansdown says: "This represents a catastrophic additional tax charge for high earning individuals such as company directors, and any employer running a final salary scheme is likely to give up at this point.

"Your senior execs have gone from enjoying a tax break on their own contributions to a situation where they not only lose their own tax relief, they also have to pay tax on their employer's contributions. It is a fair bet that many of them will just shut their final salary scheme down and walk away."

Mark Edwards, senior director taxation at the Association of British Insurers agreed the new rules, if implemented, could force even more final salary schemes to close: "If this is going to add even more complexity to administering a scheme, there will be further impetus to close schemes."

He calculates that the average cost per individual would be £10,650 if the legislation to enable this were already in place, based on HMRC data showing the number of taxpayers earning more than £150,000 (a total of 291,000) and the £3.1bn anticipated gain to the exchequer.

Alex Waite, head of corporate consulting at Lane Clark & Peacock, agreed: "The 'tax efficiency calculations' will become rather stark and we anticipate that many high earners will be advised that it is simply tax inefficient for them to make any long term pension saving at all.

"Looking ahead they should therefore consider opting out of company pension arrangements from 2011 and seek to negotiate alternative packages with remuneration committees."